| Message ID | 20180807091655.32472-3-famz@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | Fix aio_notify_accept() | expand |
On 07/08/2018 11:16, Fam Zheng wrote: > main thread IO Thread > =============================================================== > bdrv_drained_begin() > aio_disable_external(ctx) > aio_poll(ctx, true) > ctx->notify_me += 2 > ppoll() /* blocked */ > ... > bdrv_drained_end() > ... > aio_notify() > ... > bdrv_set_aio_context() > aio_poll(ctx, false) > [1] aio_notify_accept(ctx) > /* Hang! */ Should ppoll() rather be after [1]? Otherwise the new commit message and patches look great. > aio_notify() only injects an event when ctx->notify_me is set, > correspondingly aio_notify_accept() is only useful when ctx->notify_me > _was_ set. Very good point. (Please Cc qemu-stable on the second patch too). Paolo
On Tue, 08/07 12:15, Paolo Bonzini wrote: > On 07/08/2018 11:16, Fam Zheng wrote: > > main thread IO Thread > > =============================================================== > > bdrv_drained_begin() > > aio_disable_external(ctx) > > aio_poll(ctx, true) > > ctx->notify_me += 2 > > ppoll() /* blocked */ > > ... > > bdrv_drained_end() > > ... > > aio_notify() [2] ^^^^^ > > ... > > bdrv_set_aio_context() > > aio_poll(ctx, false) > > [1] aio_notify_accept(ctx) > > /* Hang! */ > > Should ppoll() rather be after [1]? Otherwise the new commit message > and patches look great. Good point. They race and I think aio_notify_accept() is indeed done before ppoll() starts its waiting. I will finish testing and send v3. Fam > > > aio_notify() only injects an event when ctx->notify_me is set, > > correspondingly aio_notify_accept() is only useful when ctx->notify_me > > _was_ set. > > Very good point. > > (Please Cc qemu-stable on the second patch too). > > Paolo
diff --git a/util/aio-posix.c b/util/aio-posix.c index b5c7f463aa..b5c609b68b 100644 --- a/util/aio-posix.c +++ b/util/aio-posix.c @@ -591,6 +591,7 @@ bool aio_poll(AioContext *ctx, bool blocking) * so disable the optimization now. */ if (blocking) { + assert(in_aio_context_home_thread(ctx)); atomic_add(&ctx->notify_me, 2); } @@ -633,6 +634,7 @@ bool aio_poll(AioContext *ctx, bool blocking) if (blocking) { atomic_sub(&ctx->notify_me, 2); + aio_notify_accept(ctx); } /* Adjust polling time */ @@ -676,8 +678,6 @@ bool aio_poll(AioContext *ctx, bool blocking) } } - aio_notify_accept(ctx); - /* if we have any readable fds, dispatch event */ if (ret > 0) { for (i = 0; i < npfd; i++) { diff --git a/util/aio-win32.c b/util/aio-win32.c index e676a8d9b2..c58957cc4b 100644 --- a/util/aio-win32.c +++ b/util/aio-win32.c @@ -373,11 +373,12 @@ bool aio_poll(AioContext *ctx, bool blocking) ret = WaitForMultipleObjects(count, events, FALSE, timeout); if (blocking) { assert(first); + assert(in_aio_context_home_thread(ctx)); atomic_sub(&ctx->notify_me, 2); + aio_notify_accept(ctx); } if (first) { - aio_notify_accept(ctx); progress |= aio_bh_poll(ctx); first = false; }
An aio_notify() pairs with an aio_notify_accept(). The former should happen in the main thread or a vCPU thread, and the latter should be done in the IOThread. There is one rare case that the main thread or vCPU thread may "steal" the aio_notify() event just raised by itself, in bdrv_set_aio_context() [1]. The sequence is like this: main thread IO Thread =============================================================== bdrv_drained_begin() aio_disable_external(ctx) aio_poll(ctx, true) ctx->notify_me += 2 ppoll() /* blocked */ ... bdrv_drained_end() ... aio_notify() ... bdrv_set_aio_context() aio_poll(ctx, false) [1] aio_notify_accept(ctx) /* Hang! */ [1] is problematic. It will clear the ctx->notifier event so that the blocked ppoll() will not return. (For the curious, this bug was noticed when booting a number of VMs simultaneously in RHV. One or two of the VMs will hit this race condition, making the VIRTIO device unresponsive to I/O commands. When it hangs, Seabios is busy waiting for a read request to complete (read MBR), right after initializing the virtio-blk-pci device, using 100% guest CPU. See also https://bugzilla.redhat.com/show_bug.cgi?id=1562750 for the original bug analysis.) aio_notify() only injects an event when ctx->notify_me is set, correspondingly aio_notify_accept() is only useful when ctx->notify_me _was_ set. Move the call to it into the "blocking" branch. This will effectively skip [1] and fix the hang. Furthermore, blocking aio_poll is only allowed on home thread (in_aio_context_home_thread), because otherwise two blocking aio_poll()'s can steal each other's ctx->notifier event and cause hanging just like described above. Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com> --- util/aio-posix.c | 4 ++-- util/aio-win32.c | 3 ++- 2 files changed, 4 insertions(+), 3 deletions(-)