sched/completion: Avoid unnecessary stack allocation for COMPLETION_INITIALIZER_ONSTACK()
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@arndb.de> Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Arnd Bergmann <arnd@arndb.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Byungchul Park <byungchul.park@lge.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Nicholas Piggin <npiggin@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: walken@google.com Cc: willy@infradead.org Link: http://lkml.kernel.org/r/20170823152542.5150-3-boqun.feng@gmail.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -74,7 +74,7 @@ static inline void complete_release_commit(struct completion *x) {}
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#endif
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#define COMPLETION_INITIALIZER_ONSTACK(work) \
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({ init_completion(&work); work; })
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(*({ init_completion(&work); &work; }))
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/**
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* DECLARE_COMPLETION - declare and initialize a completion structure
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