Message ID | 1323367477-21685-6-git-send-email-kamal@canonical.com |
---|---|
State | Not Applicable, archived |
Headers | show |
On Thu 08-12-11 10:04:35, Kamal Mostafa wrote: > From: Valerie Aurora <val@vaaconsulting.com> > > Use trylock in try_to_writeback_inodes_sb to avoid read-write > deadlocks that could be triggered by freeze. Christoph asked about what is the exact deadlock this patch tries to fix. I don't think you answered that. So can you elaborate please? Is it somehow connected with the fact that ext4 calls try_to_writeback_inodes_sb() with i_mutex held? Honza > BugLink: https://bugs.launchpad.net/bugs/897421 > Signed-off-by: Valerie Aurora <val@vaaconsulting.com> > Cc: Kamal Mostafa <kamal@canonical.com> > Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com> > [kamal@canonical.com: patch restructure] > Signed-off-by: Kamal Mostafa <kamal@canonical.com> > --- > fs/fs-writeback.c | 13 ++++++++----- > 1 files changed, 8 insertions(+), 5 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index ea89b3f..3a80f1b 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb); > * try_to_writeback_inodes_sb - start writeback if none underway > * @sb: the superblock > * > - * Invoke writeback_inodes_sb if no writeback is currently underway. > - * Returns 1 if writeback was started, 0 if not. > + * Invoke writeback_inodes_sb if no writeback is currently underway > + * and no one else holds the s_umount lock. Returns 1 if writeback > + * was started, 0 if not. > */ > int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason) > { > @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb); > * @sb: the superblock > * @nr: the number of pages to write > * > - * Invoke writeback_inodes_sb if no writeback is currently underway. > - * Returns 1 if writeback was started, 0 if not. > + * Invoke writeback_inodes_sb if no writeback is currently underway > + * and no one else holds the s_umount lock. Returns 1 if writeback > + * was started, 0 if not. > */ > int try_to_writeback_inodes_sb_nr(struct super_block *sb, > unsigned long nr, > enum wb_reason reason) > { > if (!writeback_in_progress(sb->s_bdi)) { > - down_read(&sb->s_umount); > + if (!down_read_trylock(&sb->s_umount)) > + return 0; > if (nr == 0) > writeback_inodes_sb(sb, reason); > else > -- > 1.7.5.4 >
On Fri, 2012-01-06 at 01:35 +0100, Jan Kara wrote: > On Thu 08-12-11 10:04:35, Kamal Mostafa wrote: > > From: Valerie Aurora <val@vaaconsulting.com> > > > > Use trylock in try_to_writeback_inodes_sb to avoid read-write > > deadlocks that could be triggered by freeze. > Christoph asked about what is the exact deadlock this patch tries to fix. > I don't think you answered that. So can you elaborate please? Is it somehow > connected with the fact that ext4 calls try_to_writeback_inodes_sb() with > i_mutex held? > > Honza This was discussed in the thread http://www.spinics.net/lists/linux-fsdevel/msg48754.html Summarizing... Jan> What's exactly the deadlock trylock protects from here? Jan> Or is it just an optimization? Val> The trylock is an optimization Dave Chinner suggested. The first Val> version I wrote acquired the lock and then checked vfs_is_frozen(). Dave> It's not so much an optimisation, but the general case of avoiding Dave> read-write deadlocks such that freezing can trigger. I think remount Dave> can trigger the same deadlock as freezing, so the trylock avoids Dave> both deadlock cases rather than just working around the freeze Dave> problem.... -Kamal > > BugLink: https://bugs.launchpad.net/bugs/897421 > > Signed-off-by: Valerie Aurora <val@vaaconsulting.com> > > Cc: Kamal Mostafa <kamal@canonical.com> > > Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com> > > [kamal@canonical.com: patch restructure] > > Signed-off-by: Kamal Mostafa <kamal@canonical.com> > > --- > > fs/fs-writeback.c | 13 ++++++++----- > > 1 files changed, 8 insertions(+), 5 deletions(-) > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > index ea89b3f..3a80f1b 100644 > > --- a/fs/fs-writeback.c > > +++ b/fs/fs-writeback.c > > @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb); > > * try_to_writeback_inodes_sb - start writeback if none underway > > * @sb: the superblock > > * > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > - * Returns 1 if writeback was started, 0 if not. > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > + * was started, 0 if not. > > */ > > int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason) > > { > > @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb); > > * @sb: the superblock > > * @nr: the number of pages to write > > * > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > - * Returns 1 if writeback was started, 0 if not. > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > + * was started, 0 if not. > > */ > > int try_to_writeback_inodes_sb_nr(struct super_block *sb, > > unsigned long nr, > > enum wb_reason reason) > > { > > if (!writeback_in_progress(sb->s_bdi)) { > > - down_read(&sb->s_umount); > > + if (!down_read_trylock(&sb->s_umount)) > > + return 0; > > if (nr == 0) > > writeback_inodes_sb(sb, reason); > > else > > -- > > 1.7.5.4 > >
On Wed, 11 Jan 2012, Kamal Mostafa wrote: > On Fri, 2012-01-06 at 01:35 +0100, Jan Kara wrote: > > On Thu 08-12-11 10:04:35, Kamal Mostafa wrote: > > > From: Valerie Aurora <val@vaaconsulting.com> > > > > > > Use trylock in try_to_writeback_inodes_sb to avoid read-write > > > deadlocks that could be triggered by freeze. > > > Christoph asked about what is the exact deadlock this patch tries to fix. > > I don't think you answered that. So can you elaborate please? Is it somehow > > connected with the fact that ext4 calls try_to_writeback_inodes_sb() with > > i_mutex held? > > > > Honza > > This was discussed in the thread > http://www.spinics.net/lists/linux-fsdevel/msg48754.html > Summarizing... > > Jan> What's exactly the deadlock trylock protects from here? > Jan> Or is it just an optimization? > > Val> The trylock is an optimization Dave Chinner suggested. The first > Val> version I wrote acquired the lock and then checked vfs_is_frozen(). > > Dave> It's not so much an optimisation, but the general case of avoiding > Dave> read-write deadlocks such that freezing can trigger. I think remount > Dave> can trigger the same deadlock as freezing, so the trylock avoids > Dave> both deadlock cases rather than just working around the freeze > Dave> problem.... > > -Kamal As I wrote in https://www.redhat.com/archives/dm-devel/2011-November/msg00151.html , down_read_trylock doesn't fix the freeze deadlock. Think of this sequence: Process 1 (freezing) down_write(&sb->s_umount); set the filesystem to frozen state up_write(&sb->s_umount); Process 2 (executing the code from the patch) down_read_trylock(&sb->s_umount); - succeeds, because s_umount is not held writeback_inodes_sb(sb, reason); - waits, because the filesystem is frozen Process 1 (unfreezing) down_write(&sb->s_umount); - deadlock (process 1 is waiting for process 2 to drop the lock; process 2 is waiting for process 1 to unfreeze). See the patch at https://www.redhat.com/archives/dm-devel/2011-November/msg00151.html , it has a different approach and it avoids the mentined freeze deadlock. Mikulas > > > BugLink: https://bugs.launchpad.net/bugs/897421 > > > Signed-off-by: Valerie Aurora <val@vaaconsulting.com> > > > Cc: Kamal Mostafa <kamal@canonical.com> > > > Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com> > > > [kamal@canonical.com: patch restructure] > > > Signed-off-by: Kamal Mostafa <kamal@canonical.com> > > > --- > > > fs/fs-writeback.c | 13 ++++++++----- > > > 1 files changed, 8 insertions(+), 5 deletions(-) > > > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > > index ea89b3f..3a80f1b 100644 > > > --- a/fs/fs-writeback.c > > > +++ b/fs/fs-writeback.c > > > @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb); > > > * try_to_writeback_inodes_sb - start writeback if none underway > > > * @sb: the superblock > > > * > > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > > - * Returns 1 if writeback was started, 0 if not. > > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > > + * was started, 0 if not. > > > */ > > > int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason) > > > { > > > @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb); > > > * @sb: the superblock > > > * @nr: the number of pages to write > > > * > > > - * Invoke writeback_inodes_sb if no writeback is currently underway. > > > - * Returns 1 if writeback was started, 0 if not. > > > + * Invoke writeback_inodes_sb if no writeback is currently underway > > > + * and no one else holds the s_umount lock. Returns 1 if writeback > > > + * was started, 0 if not. > > > */ > > > int try_to_writeback_inodes_sb_nr(struct super_block *sb, > > > unsigned long nr, > > > enum wb_reason reason) > > > { > > > if (!writeback_in_progress(sb->s_bdi)) { > > > - down_read(&sb->s_umount); > > > + if (!down_read_trylock(&sb->s_umount)) > > > + return 0; > > > if (nr == 0) > > > writeback_inodes_sb(sb, reason); > > > else > > > -- > > > 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index ea89b3f..3a80f1b 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb); * try_to_writeback_inodes_sb - start writeback if none underway * @sb: the superblock * - * Invoke writeback_inodes_sb if no writeback is currently underway. - * Returns 1 if writeback was started, 0 if not. + * Invoke writeback_inodes_sb if no writeback is currently underway + * and no one else holds the s_umount lock. Returns 1 if writeback + * was started, 0 if not. */ int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason) { @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb); * @sb: the superblock * @nr: the number of pages to write * - * Invoke writeback_inodes_sb if no writeback is currently underway. - * Returns 1 if writeback was started, 0 if not. + * Invoke writeback_inodes_sb if no writeback is currently underway + * and no one else holds the s_umount lock. Returns 1 if writeback + * was started, 0 if not. */ int try_to_writeback_inodes_sb_nr(struct super_block *sb, unsigned long nr, enum wb_reason reason) { if (!writeback_in_progress(sb->s_bdi)) { - down_read(&sb->s_umount); + if (!down_read_trylock(&sb->s_umount)) + return 0; if (nr == 0) writeback_inodes_sb(sb, reason); else