An XFS workload showed up a bug in the lockless pagecache patch. Basically it
would go into an "infinite" loop, although it would sometimes be able to break
out of the loop! The reason is a missing compiler barrier in the "increment
reference count unless it was zero" case of the lockless pagecache protocol in
the gang lookup functions.
This would cause the compiler to use a cached value of struct page pointer to
retry the operation with, rather than reload it. So the page might have been
removed from pagecache and freed (refcount==0) but the lookup would not correctly
notice the page is no longer in pagecache, and keep attempting to increment the
refcount and failing, until the page gets reallocated for something else. This
isn't a data corruption because the condition will be detected if the page has
been reallocated. However it can result in a lockup.
Linus points out that ACCESS_ONCE is also required in that pointer load, even
if it's absence is not causing a bug on our particular build. The most general
way to solve this is just to put an rcu_dereference in radix_tree_deref_slot.
Assembly of find_get_pages,
before:
.L220:
movq (%rbx), %rax #* ivtmp.1162, tmp82
movq (%rax), %rdi #, prephitmp.1149
.L218:
testb $1, %dil #, prephitmp.1149
jne .L217 #,
testq %rdi, %rdi # prephitmp.1149
je .L203 #,
cmpq $-1, %rdi #, prephitmp.1149
je .L217 #,
movl 8(%rdi), %esi # <variable>._count.counter, c
testl %esi, %esi # c
je .L218 #,
after:
.L212:
movq (%rbx), %rax #* ivtmp.1109, tmp81
movq (%rax), %rdi #, ret
testb $1, %dil #, ret
jne .L211 #,
testq %rdi, %rdi # ret
je .L197 #,
cmpq $-1, %rdi #, ret
je .L211 #,
movl 8(%rdi), %esi # <variable>._count.counter, c
testl %esi, %esi # c
je .L212 #,
(notice the obvious infinite loop in the first example, if page->count remains 0)
Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
e8c82c2e23