On Wed, Sep 25, 2024 at 01:59:06PM +0200, Mathieu Desnoyers wrote: > On 2024-09-25 12:45, Boqun Feng wrote: > > On Wed, Sep 25, 2024 at 12:11:52PM +0200, Jonas Oberhauser wrote: > > > > > > > > > Am 9/25/2024 um 12:02 PM schrieb Boqun Feng: > > > > Hi Jonas, > > > > > > > > Of > > > > course, if we are really worried about compilers being too "smart" > > > > > > Ah, I see you know me better and better... > > > > > > > we can always do the comparison in asm code, then compilers don't know > > > > anything of the equality between 'ptr' and 'head - head_offset'. > > > Yes, but then a simple compiler barrier between the comparison and returning > > > ptr would also do the trick, right? And maybe easier on the eyes. > > > > > > > The thing about putting a compiler barrier is that it will prevent all > > compiler reorderings, and some of the reordering may contribute to > > better codegen. (I know in this case, we have a smp_mb(), but still > > compilers can move unrelated code upto the second load for optimization > > purpose). Asm comparison is cheaper in this way. But TBH, compilers > > should provide a way to compare pointer values without using the result > > for pointer equality proof, if "convert to unsigned long" doesn't work, > > some other ways should work. > > > > Based on Documentation/RCU/rcu_dereference.rst : > > - Be very careful about comparing pointers obtained from > rcu_dereference() against non-NULL values. As Linus Torvalds > explained, if the two pointers are equal, the compiler could > substitute the pointer you are comparing against for the pointer > obtained from rcu_dereference(). For example:: > > p = rcu_dereference(gp); > if (p == &default_struct) > do_default(p->a); > > Because the compiler now knows that the value of "p" is exactly > the address of the variable "default_struct", it is free to > transform this code into the following:: > > p = rcu_dereference(gp); > if (p == &default_struct) > do_default(default_struct.a); > > On ARM and Power hardware, the load from "default_struct.a" > can now be speculated, such that it might happen before the > rcu_dereference(). This could result in bugs due to misordering. > > So I am not only concerned about compiler proofs here, as it appears > that the speculation done by the CPU can also cause issues on some > architectures. > Note that the issue is caused by compiler replacing one pointer with the other, CPU speculation won't cause the issue solely, because all architectures Linux supports honor address dependencies (except for Alpha, but on Alpha, READ_ONCE() has a smp_mb() after the load). That is: if the compiler generates code as the code is written, there should be no problem. Regards, Boqun > Thanks, > > Mathieu > > > Regards, > > Boqun > > > > > > > > Have fun, > > > jonas > > > > > -- > Mathieu Desnoyers > EfficiOS Inc. > https://www.efficios.com >
