From 55bccf1f93e4bf1b3209cc8648ab53f10f4601a5 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 29 Jul 2021 16:17:20 +0200
Subject: [PATCH] Documentation/atomic_t: Document forward progress expectations
Git-commit: 55bccf1f93e4bf1b3209cc8648ab53f10f4601a5
Patch-mainline: v5.15-rc1
References: bsc#1190137
Add a few words on forward progress; there's been quite a bit of
confusion on the subject.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lkml.kernel.org/r/YQK9ziyogxTH0m9H@hirez.programming.kicks-ass.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
Documentation/atomic_t.txt | 53 ++++++++++++++++++++++++++++++++++++++
1 file changed, 53 insertions(+)
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index a9c1e2b39b15..0f1ffa03db09 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -312,3 +312,56 @@ Usage:
NB. try_cmpxchg() also generates better code on some platforms (notably x86)
where the function more closely matches the hardware instruction.
+
+
+FORWARD PROGRESS
+----------------
+
+In general strong forward progress is expected of all unconditional atomic
+operations -- those in the Arithmetic and Bitwise classes and xchg(). However
+a fair amount of code also requires forward progress from the conditional
+atomic operations.
+
+Specifically 'simple' cmpxchg() loops are expected to not starve one another
+indefinitely. However, this is not evident on LL/SC architectures, because
+while an LL/SC architecure 'can/should/must' provide forward progress
+guarantees between competing LL/SC sections, such a guarantee does not
+transfer to cmpxchg() implemented using LL/SC. Consider:
+
+ old = atomic_read(&v);
+ do {
+ new = func(old);
+ } while (!atomic_try_cmpxchg(&v, &old, new));
+
+which on LL/SC becomes something like:
+
+ old = atomic_read(&v);
+ do {
+ new = func(old);
+ } while (!({
+ volatile asm ("1: LL %[oldval], %[v]\n"
+ " CMP %[oldval], %[old]\n"
+ " BNE 2f\n"
+ " SC %[new], %[v]\n"
+ " BNE 1b\n"
+ "2:\n"
+ : [oldval] "=&r" (oldval), [v] "m" (v)
+ : [old] "r" (old), [new] "r" (new)
+ : "memory");
+ success = (oldval == old);
+ if (!success)
+ old = oldval;
+ success; }));
+
+However, even the forward branch from the failed compare can cause the LL/SC
+to fail on some architectures, let alone whatever the compiler makes of the C
+loop body. As a result there is no guarantee what so ever the cacheline
+containing @v will stay on the local CPU and progress is made.
+
+Even native CAS architectures can fail to provide forward progress for their
+primitive (See Sparc64 for an example).
+
+Such implementations are strongly encouraged to add exponential backoff loops
+to a failed CAS in order to ensure some progress. Affected architectures are
+also strongly encouraged to inspect/audit the atomic fallbacks, refcount_t and
+their locking primitives.
--
2.26.2