In theory, COMPLETION_INITIALIZER_ONSTACK() should never affect the
stack allocation of the caller. However, on some compilers, a temporary
structure was allocated for the return value of
COMPLETION_INITIALIZER_ONSTACK(), for example in write_journal() with
LOCKDEP_COMPLETIONS=y(gcc is 7.1.1):
io_comp.comp = COMPLETION_INITIALIZER_ONSTACK(io_comp.comp);
2462: e8 00 00 00 00 callq 2467 <write_journal+0x47>
2467: 48 8d 85 80 fd ff ff lea -0x280(%rbp),%rax
246e: 48 c7 c6 00 00 00 00 mov $0x0,%rsi
2475: 48 c7 c2 00 00 00 00 mov $0x0,%rdx
x->done = 0;
247c: c7 85 90 fd ff ff 00 movl $0x0,-0x270(%rbp)
2483: 00 00 00
init_waitqueue_head(&x->wait);
2486: 48 8d 78 18 lea 0x18(%rax),%rdi
248a: e8 00 00 00 00 callq 248f <write_journal+0x6f>
if (commit_start + commit_sections <= ic->journal_sections) {
248f: 41 8b 87 a8 00 00 00 mov 0xa8(%r15),%eax
io_comp.comp = COMPLETION_INITIALIZER_ONSTACK(io_comp.comp);
2496: 48 8d bd e8 f9 ff ff lea -0x618(%rbp),%rdi
249d: 48 8d b5 90 fd ff ff lea -0x270(%rbp),%rsi
24a4: b9 17 00 00 00 mov $0x17,%ecx
24a9: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi)
if (commit_start + commit_sections <= ic->journal_sections) {
24ac: 41 39 c6 cmp %eax,%r14d
io_comp.comp = COMPLETION_INITIALIZER_ONSTACK(io_comp.comp);
24af: 48 8d bd 90 fd ff ff lea -0x270(%rbp),%rdi
24b6: 48 8d b5 e8 f9 ff ff lea -0x618(%rbp),%rsi
24bd: b9 17 00 00 00 mov $0x17,%ecx
24c2: f3 48 a5 rep movsq %ds:(%rsi),%es:(%rdi)
We can obviously see the temporary structure allocated, and the compiler
also does two meaningless memcpy with "rep movsq".
And according to:
https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html#Statement-Exprs
The return value of a statement expression is returned by value, so the
temporary variable is created in COMPLETION_INITIALIZER_ONSTACK(), and
that's why the temporary structures are allocted.
To fix this, make the brace block in COMPLETION_INITIALIZER_ONSTACK()
return a pointer and dereference it outside the block rather than return
the whole structure, in this way, we are able to teach the compiler not
to do the unnecessary stack allocation.
This could also reduce the stack size even if !LOCKDEP, for example in
write_journal(), compiled with gcc 7.1.1, the result of command:
objdump -d drivers/md/dm-integrity.o | ./scripts/checkstack.pl x86
before:
0x0000246a write_journal [dm-integrity.o]: 696
after:
0x00002b7a write_journal [dm-integrity.o]: 296
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
include/linux/completion.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/completion.h b/include/linux/completion.h
index 791f053f28b7..cae5400022a3 100644
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -74,7 +74,7 @@ static inline void complete_release_commit(struct completion *x) {}
#endif
#define COMPLETION_INITIALIZER_ONSTACK(work) \
- ({ init_completion(&work); work; })
+ (*({ init_completion(&work); &work; }))
/**
* DECLARE_COMPLETION - declare and initialize a completion structure
--
2.14.1
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