Re: [musl] Illegal killlock skipping when transitioning to single-threaded state

[Rich Felker <dalias@libc.org>]

On Mon, Oct 03, 2022 at 10:58:32PM -0400, Rich Felker wrote:
> On Mon, Oct 03, 2022 at 11:27:05PM +0200, Szabolcs Nagy wrote:
> > * Szabolcs Nagy <nsz@port70.net> [2022-10-03 15:26:15 +0200]:
> > 
> > > * Alexey Izbyshev <izbyshev@ispras.ru> [2022-10-03 09:16:03 +0300]:
> > > > On 2022-09-19 18:29, Rich Felker wrote:
> > > > > On Wed, Sep 07, 2022 at 03:46:53AM +0300, Alexey Izbyshev wrote:
> > > ...
> > > > > > Reordering the "libc.need_locks = -1" assignment and
> > > > > > UNLOCK(E->killlock) and providing a store barrier between them
> > > > > > should fix the issue.
> > > > > 
> > > > > I think this all sounds correct. I'm not sure what you mean by a store
> > > > > barrier between them, since all lock and unlock operations are already
> > > > > full barriers.
> > > > > 
> > > > 
> > > > Before sending the report I tried to infer the intended ordering semantics
> > > > of LOCK/UNLOCK by looking at their implementations. For AArch64, I didn't
> > > > see why they would provide a full barrier (my reasoning is below), so I
> > > > concluded that probably acquire/release semantics was intended in general
> > > > and suggested an extra store barrier to prevent hoisting of "libc.need_locks
> > > > = -1" store spelled after UNLOCK(E->killlock) back into the critical
> > > > section.
> > > > 
> > > > UNLOCK is implemented via a_fetch_add(). On AArch64, it is a simple
> > > > a_ll()/a_sc() loop without extra barriers, and a_ll()/a_sc() are implemented
> > > > via load-acquire/store-release instructions. Therefore, if we consider a
> > > > LOCK/UNLOCK critical section containing only plain loads and stores, (a) any
> > > > such memory access can be reordered with the initial ldaxr in UNLOCK, and
> > > > (b) any plain load following UNLOCK can be reordered with stlxr (assuming
> > > > the processor predicts that stlxr succeeds), and further, due to (a), with
> > > > any memory access inside the critical section. Therefore, UNLOCK is not full
> > > > barrier. Is this right?
> > > 
> > > i dont think this is right.
> > 
> > 
> > i think i was wrong and you are right.
> > 
> > so with your suggested swap of UNLOCK(killlock) and need_locks=-1 and
> > starting with 'something == 0' the exiting E and remaining R threads:
> > 
> > E:something=1      // protected by killlock
> > E:UNLOCK(killlock)
> > E:need_locks=-1
> > 
> > R:LOCK(unrelated)  // reads need_locks == -1
> > R:need_locks=0
> > R:UNLOCK(unrelated)
> > R:LOCK(killlock)   // does not lock
> > R:read something   // can it be 0 ?
> > 
> > and here something can be 0 (ie. not protected by killlock) on aarch64
> > because
> > 
> > T1
> > 	something=1
> > 	ldaxr ... killlock
> > 	stlxr ... killlock
> > 	need_locks=-1
> > 
> > T2
> > 	x=need_locks
> > 	ldaxr ... unrelated
> > 	stlxr ... unrelated
> > 	y=something
> > 
> > can end with x==-1 and y==0.
> > 
> > and to fix it, both a_fetch_add and a_cas need an a_barrier.
> > 
> > i need to think how to support such lock usage on aarch64
> > without adding too many dmb.
> 
> OK, after reading a lot more, I think I'm starting to get what you're
> saying. Am I correct in my understanding that the problem is that the
> "R:LOCK(unrelated)" as implemented does not synchronize with the
> "E:UNLOCK(killlock)" because they're different objects?
> 
> If so, I think this would be fully solved by using __tl_sync in the
> code path that resets need_locks to 0 after observing -1, by virtue of
> providing a common object (the thread list lock) to synchronize on.
> This is the "weaker memory model friendly" approach we should probably
> strive to achieve some day.
> 
> However, all existing code in musl is written assuming what I call the
> "POSIX memory model" where the only operation is "synchronizes memory"
> and that underspecified phrase has to be interpreted as "is a full
> barrier" to admit any consistent model. Said differently, the code was
> written assuming every a_* synchronizes with every other a_*, without
> any regard for whether they act on the same objects. This likely even
> matters for how our waiter accounting works (which is probably a good
> argument for removing it and switching to waiter flags). So I think,
> if the issue as I understand it now exists, we do need to fix it. Then
> we can revisit this at some later time as part of a big project.

Forgot, I should include links to the material I've been reading:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65697
https://gcc.gnu.org/git?p=gcc.git;a=commitdiff;h=f70fb3b635f9618c6d2ee3848ba836914f7951c2
https://gcc.gnu.org/git?p=gcc.git;a=commitdiff;h=ab876106eb689947cdd8203f8ecc6e8ac38bf5ba

which is where the GCC folks seem to have encountered and fixed their
corresponding issue.

Rich