| Message ID | 20180809132259.18402-3-famz@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Fix aio_notify_accept() | expand |
Am 09.08.2018 um 15:22 hat Fam Zheng geschrieben: > 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. It's good to have this assertion now at least, but after digging into some bugs, I think in fact that any aio_poll() (even non-blocking) is only allowed in the home thread: At least one reason is that if you run it from a different thread, qemu_get_current_aio_context() returns the wrong AioContext in any callbacks called by aio_poll(). Anything else using TLS can have similar problems. One instance where this matters is fixed/worked around by Sergio's "util/async: use qemu_aio_coroutine_enter in co_schedule_bh_cb". We wouldn't even need that patch if we could make sure that aio_poll() is never called from the wrong thread. This would feel more robust. I'll fix the aio_poll() calls in drain (the AIO_WAIT_WHILE() ones are already fine, the rest by removing them). After that, bdrv_set_aio_context() is still problematic, but the rest should be okay. Hopefully we can use the tighter assertion then. Kevin
On Fri, 09/07 17:51, Kevin Wolf wrote: > Am 09.08.2018 um 15:22 hat Fam Zheng geschrieben: > > 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. > > It's good to have this assertion now at least, but after digging into > some bugs, I think in fact that any aio_poll() (even non-blocking) is > only allowed in the home thread: At least one reason is that if you run > it from a different thread, qemu_get_current_aio_context() returns the > wrong AioContext in any callbacks called by aio_poll(). Anything else > using TLS can have similar problems. > > One instance where this matters is fixed/worked around by Sergio's > "util/async: use qemu_aio_coroutine_enter in co_schedule_bh_cb". We > wouldn't even need that patch if we could make sure that aio_poll() is > never called from the wrong thread. This would feel more robust. > > I'll fix the aio_poll() calls in drain (the AIO_WAIT_WHILE() ones are > already fine, the rest by removing them). After that, > bdrv_set_aio_context() is still problematic, but the rest should be > okay. Hopefully we can use the tighter assertion then. Fully agree with you. Fam
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 ... bdrv_drained_end() ... aio_notify() ... bdrv_set_aio_context() aio_poll(ctx, false) [1] aio_notify_accept(ctx) ppoll() /* 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. Cc: qemu-stable@nongnu.org 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(-)