Messages in this thread | | | Date | Mon, 5 Feb 2024 15:56:03 -0800 | From | Ira Weiny <> | Subject | RE: [PATCH 1/2 v2] cleanup: Add cond_guard() to conditional guards |
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Dan Williams wrote: > Ira Weiny wrote: > > Dan Williams wrote: > > > Fabio M. De Francesco wrote: > > > > Add cond_guard() macro to conditional guards. > > > > > > > > cond_guard() is a guard to be used with the conditional variants of locks, > > > > like down_read_trylock() or mutex_lock_interruptible(). > > > > > > > > It takes a statement (or more statements in a block) that is passed to its > > > > second argument. That statement (or block) is executed if waiting for a > > > > lock is interrupted or if a _trylock() fails in case of contention. > > > > > > > > Usage example: > > > > > > > > cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore); > > > > > > > > Consistently with the other guards, locks are unlocked at the exit of the > > > > scope where cond_guard() is called. > > > > > > > > Cc: Peter Zijlstra <peterz@infradead.org> > > > > Suggested-by: Dan Williams <dan.j.williams@intel.com> > > > > Suggested-by: Ira Weiny <ira.weiny@intel.com> > > > > Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com> > > > > --- > > > > include/linux/cleanup.h | 14 ++++++++++++++ > > > > 1 file changed, 14 insertions(+) > > > > > > > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h > > > > index c2d09bc4f976..88af56600325 100644 > > > > --- a/include/linux/cleanup.h > > > > +++ b/include/linux/cleanup.h > > > > @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \ > > > > * an anonymous instance of the (guard) class, not recommended for > > > > * conditional locks. > > > > * > > > > + * cond_guard(name, fail, args...): > > > > + * a guard to be used with the conditional variants of locks, like > > > > + * down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more > > > > + * statements that are executed when waiting for a lock is interrupted or > > > > + * when a _trylock() fails in case of contention. > > > > + * > > > > + * Example: > > > > + * > > > > + * cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore); > > > > > > That _fail argument likely needs to be a statement expression for the > > > multi-statement case. > > > > You mean ({ ... }) as discussed here? > > > > https://lore.kernel.org/all/65c1578c76def_37447929456@iweiny-mobl.notmuch/ > > Yes. > > > > > + * > > > > * scoped_guard (name, args...) { }: > > > > * similar to CLASS(name, scope)(args), except the variable (with the > > > > * explicit name 'scope') is declard in a for-loop such that its scope is > > > > @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \ > > > > > > > > #define __guard_ptr(_name) class_##_name##_lock_ptr > > > > > > > > +#define cond_guard(_name, _fail, args...) \ > > > > + CLASS(_name, scope)(args); \ > > > > + if (!__guard_ptr(_name)(&scope)) _fail > > > > > > No, as I stated before this is broken for usages of: > > > > > > if () cond_guard() else if () > > > > > > The 'else' in the definition is critical, this builds for me (untested): > > > > I did not test Fabios work directly but I don't understand this example. > > It seems like your suggestion does nothing useful. The cond_guard() > > becomes a single statement like... > > > > if () > > cond_guard(); > > else ... > > > > ... And can't protect anything. > > A sequence to acquire and drop a lock is sometimes a barrier semantic. > Is it typical, no, is it possible, yes. I otherwise do not understand > the need to include the subtle side effect.
I was not trying to include a subtle side effect. I was thinking that the else block would be the only block covered by the lock. I've looked at the preprocessor output again and I now see what you are saying. Also I see I was thinking incorrectly. The else will be an empty statement and the rest of the outer block will be covered by the lock...
Sorry for not seeing this before.
> > cond_guard() as defined, the ';' must be used as part of cond_guard() and > > should complete the internal macro 'if' statement. > > > > I think this would work: > > > > if () { > > cond_guard(); > > ... do locked stuff ... > > } else ... > > > > > > > > diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h > > > index 88af56600325..665407498781 100644 > > > --- a/include/linux/cleanup.h > > > +++ b/include/linux/cleanup.h > > > @@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \ > > > * > > > * Example: > > > * > > > - * cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore); > > > + * cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore); > > > * > > > * scoped_guard (name, args...) { }: > > > * similar to CLASS(name, scope)(args), except the variable (with the > > > @@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \ > > > > > > #define cond_guard(_name, _fail, args...) \ > > > CLASS(_name, scope)(args); \ > > > - if (!__guard_ptr(_name)(&scope)) _fail > > > + if (!__guard_ptr(_name)(&scope)) _fail; \ > > > > Building on what I found for scoped_cond_guard() this should be > > > > > + if (!__guard_ptr(_name)(&scope)) { _fail; } > > That's still a dangling if () statement. > > > > > And drop the else. The else needs to clearly be part of an outside if in > > your example. > > Please just rely on a statement-expression for the odd multi-statement _fail > use case and include the else in the definition to remove any room for > confusion.
Yea ok I see it now, Ira
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