From: Andrea Parri <parri.andrea@gmail.com>
To: Waiman Long <longman@redhat.com>
Cc: Prateek Sood <prsood@codeaurora.org>,
peterz@infradead.org, mingo@redhat.com, sramana@codeaurora.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load
Date: Thu, 10 Aug 2017 10:32:56 +0200 [thread overview]
Message-ID: <20170810083255.GA3913@andrea> (raw)
In-Reply-To: <680a3f82-b36b-e09f-b1db-fa23d9f21fc2@redhat.com>
On Thu, Jul 27, 2017 at 11:48:53AM -0400, Waiman Long wrote:
> On 07/26/2017 04:17 PM, Prateek Sood wrote:
> > If a spinner is present, there is a chance that the load of
> > rwsem_has_spinner() in rwsem_wake() can be reordered with
> > respect to decrement of rwsem count in __up_write() leading
> > to wakeup being missed.
> >
> > spinning writer up_write caller
> > --------------- -----------------------
> > [S] osq_unlock() [L] osq
> > spin_lock(wait_lock)
> > sem->count=0xFFFFFFFF00000001
> > +0xFFFFFFFF00000000
> > count=sem->count
> > MB
> > sem->count=0xFFFFFFFE00000001
> > -0xFFFFFFFF00000001
> > spin_trylock(wait_lock)
> > return
> > rwsem_try_write_lock(count)
> > spin_unlock(wait_lock)
> > schedule()
> >
> > Reordering of atomic_long_sub_return_release() in __up_write()
> > and rwsem_has_spinner() in rwsem_wake() can cause missing of
> > wakeup in up_write() context. In spinning writer, sem->count
> > and local variable count is 0XFFFFFFFE00000001. It would result
> > in rwsem_try_write_lock() failing to acquire rwsem and spinning
> > writer going to sleep in rwsem_down_write_failed().
> >
> > The smp_rmb() will make sure that the spinner state is
> > consulted after sem->count is updated in up_write context.
> >
> > Signed-off-by: Prateek Sood <prsood@codeaurora.org>
>
> Did you actually observe that the reordering happens?
>
> I am not sure if some architectures can actually speculatively execute
> instruction ahead of a branch and then ahead into a function call. I
> know it can happen if the function call is inlined, but rwsem_wake()
> will not be inlined into __up_read() or __up_write().
Branches/control dependencies targeting a read do not necessarily preserve
program order; this is for example the case for PowerPC and ARM.
I'd not expect more than a compiler barrier from a function call (in fact,
not even that if the function happens to be inlined).
>
> Even if that is the case, I am not sure if smp_rmb() alone is enough to
> guarantee the ordering as I think it will depend on how the
> atomic_long_sub_return_release() is implmented.
AFAICT, the pattern under discussion is MP with:
- a store-release to osq->tail(unlock) followed by a store to sem->count,
separated by a MB (from atomic_long_add_return()) on CPU0;
- a load of sem->count (for atomic_long_sub_return_release()) followed by
a load of osq->tail (rwsem_has_spinner()) on CPU1.
Thus a RMW between the two loads suffices to forbid the weak behaviour.
Andrea
>
> Cheers,
> Longman
>
next prev parent reply other threads:[~2017-08-10 8:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-26 20:17 [PATCH] rwsem: fix missed wakeup due to reordering of load Prateek Sood
2017-07-27 15:48 ` Waiman Long
2017-07-27 16:59 ` Peter Zijlstra
2017-08-10 8:32 ` Andrea Parri [this message]
2017-08-10 10:41 ` Peter Zijlstra
2017-08-10 10:44 ` Peter Zijlstra
2017-08-23 11:28 Prateek Sood
2017-08-24 11:29 ` Peter Zijlstra
2017-08-24 12:33 ` Peter Zijlstra
2017-08-24 12:52 ` Peter Zijlstra
2017-09-07 14:08 ` Prateek Sood
2017-09-07 14:30 Prateek Sood
2017-09-19 14:05 ` Andrea Parri
2017-09-20 14:52 ` Davidlohr Bueso
2017-09-20 21:17 ` Andrea Parri
2017-09-27 21:20 ` Davidlohr Bueso
2017-09-26 18:37 ` Prateek Sood
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170810083255.GA3913@andrea \
--to=parri.andrea@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=longman@redhat.com \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=prsood@codeaurora.org \
--cc=sramana@codeaurora.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).