Patchwork [v2,1/2] Introduce QLIST_INSERT_HEAD_RCU and dummy RCU wrappers.

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Submitter Harsh Prateek Bora
Date Oct. 13, 2011, 8:35 p.m.
Message ID <1318538102-6982-1-git-send-email-harsh@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/119607/
State New
Headers show

Comments

Harsh Prateek Bora - Oct. 13, 2011, 8:35 p.m.
SynthFS needs a QLIST_INSERT_HEAD_RCU to make sure list instructions are not
re-ordered and therefore avoiding a crash. There may be parallel readers which
should be allowed for lock-free access and this variant allows us to get rid 
of rwlocks used by readers.

SynthFS is a special case where we dont really need full RCU capabilities as
it doesnt allow list entry deletion but concurrent readers/writers and 
instruction re-ordering should not result in a crash.

Also, once the real rcu is available, dummy rcu macro definitions will go away
and the code will still work as expected.

This patchwork is based on inputs from Paolo Bonzini.

Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
---
 qemu-queue.h  |   12 ++++++++++++
 qemu-thread.h |    3 +++
 2 files changed, 15 insertions(+), 0 deletions(-)
Paolo Bonzini - Oct. 14, 2011, 6:49 a.m.
On 10/13/2011 10:35 PM, Harsh Prateek Bora wrote:
>
> +#define QLIST_INSERT_HEAD_RCU(head, elm, field) do {                    \
> +        (elm)->field.le_prev =&(head)->lh_first;                       \
> +        smp_wmb();                                                      \
> +        if (((elm)->field.le_next = (head)->lh_first) != NULL)          \
> +                (head)->lh_first->field.le_prev =&(elm)->field.le_next;\
> +        smp_wmb();                                                      \
> +        (head)->lh_first = (elm);                                       \
> +        smp_wmb();                                                      \
> +} while (/* CONSTCOND*/0)

Actually, looking more at it it should be more like

        (elm)->field.le_prev =&(head)->lh_first;
        (elm)->field.le_next = (head)->lh_first;
        smb_wmb(); /* fill elm before linking it */
        if ((head)->lh_first != NULL)
           (head)->lh_first->field.le_prev =&(elm)->field.le_next;
        (head)->lh_first = (elm);
        smp_wmb();

... which even saves a memory barrier.

Paolo

Patch

diff --git a/qemu-queue.h b/qemu-queue.h
index 1d07745..57234f8 100644
--- a/qemu-queue.h
+++ b/qemu-queue.h
@@ -76,6 +76,8 @@ 
  * For details on the use of these macros, see the queue(3) manual page.
  */
 
+#include "qemu-barrier.h" /* for smp_wmb() */
+
 /*
  * List definitions.
  */
@@ -122,6 +124,16 @@  struct {                                                                \
         (elm)->field.le_prev = &(head)->lh_first;                       \
 } while (/*CONSTCOND*/0)
 
+#define QLIST_INSERT_HEAD_RCU(head, elm, field) do {                    \
+        (elm)->field.le_prev = &(head)->lh_first;                       \
+        smp_wmb();                                                      \
+        if (((elm)->field.le_next = (head)->lh_first) != NULL)          \
+                (head)->lh_first->field.le_prev = &(elm)->field.le_next;\
+        smp_wmb();                                                      \
+        (head)->lh_first = (elm);                                       \
+        smp_wmb();                                                      \
+} while (/* CONSTCOND*/0)
+
 #define QLIST_REMOVE(elm, field) do {                                   \
         if ((elm)->field.le_next != NULL)                               \
                 (elm)->field.le_next->field.le_prev =                   \
diff --git a/qemu-thread.h b/qemu-thread.h
index 0a73d50..e008b60 100644
--- a/qemu-thread.h
+++ b/qemu-thread.h
@@ -19,6 +19,9 @@  void qemu_mutex_lock(QemuMutex *mutex);
 int qemu_mutex_trylock(QemuMutex *mutex);
 void qemu_mutex_unlock(QemuMutex *mutex);
 
+#define rcu_read_lock() do { } while (0)
+#define rcu_read_unlock() do { } while (0)
+
 void qemu_cond_init(QemuCond *cond);
 void qemu_cond_destroy(QemuCond *cond);