On Thu, Jan 02, 2014 at 12:33:20PM -0800, Paul E. McKenney wrote: > Hello! > > From what I can see, the Linux-kernel's SLAB, SLOB, and SLUB memory > allocators would deal with the following sort of race: > > A. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(gp) = r1; > > CPU 1: r2 = ACCESS_ONCE(gp); if (r2) kfree(r2); > > However, my guess is that this should be considered an accident of the > current implementation rather than a feature. The reason for this is > that I cannot see how you would usefully do (A) above without also allowing > (B) and (C) below, both of which look to me to be quite destructive: (A) only seems OK if "gp" is guaranteed to be NULL beforehand, *and* if no other CPUs can possibly do what CPU 1 is doing in parallel. Even then, it seems questionable how this could ever be used successfully in practice. This seems similar to the TCP simultaneous-SYN case: theoretically possible, absurd in practice. > B. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(shared_x) = r1; > > CPU 1: r2 = ACCESS_ONCE(shared_x); if (r2) kfree(r2); > > CPU 2: r3 = ACCESS_ONCE(shared_x); if (r3) kfree(r3); > > This results in the memory being on two different freelists. That's a straightforward double-free bug. You need some kind of synchronization there to ensure that only one call to kfree occurs. > C. CPU 0: r1 = kmalloc(...); ACCESS_ONCE(shared_x) = r1; > > CPU 1: r2 = ACCESS_ONCE(shared_x); r2->a = 1; r2->b = 2; > > CPU 2: r3 = ACCESS_ONCE(shared_x); if (r3) kfree(r3); > > CPU 3: r4 = kmalloc(...); r4->s = 3; r4->t = 4; > > This results in the memory being used by two different CPUs, > each of which believe that they have sole access. This is not OK either: CPU 2 has called kfree on a pointer that CPU 1 still considers alive, and again, the CPUs haven't used any form of synchronization to prevent that. > But I thought I should ask the experts. > > So, am I correct that kernel hackers are required to avoid "drive-by" > kfree()s of kmalloc()ed memory? Don't kfree things that are in use, and synchronize to make sure all CPUs agree about "in use", yes. > PS. To the question "Why would anyone care about (A)?", then answer > is "Inquiring programming-language memory-model designers want > to know." I find myself wondering about the original form of the question, since I'd hope that programming-languge memory-model designers would understand the need for synchronization around reclaiming memory. - Josh Triplett -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>