Messages in this thread | | | Date | Fri, 26 Jan 2018 21:39:12 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [RFC 1/2] arm: cacheflush syscall: process only pages that are in the memory |
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On Fri, Jan 26, 2018 at 02:30:47PM +0100, Marek Szyprowski wrote: > Hi Russell, > > On 2018-01-26 12:32, Russell King - ARM Linux wrote: > >On Fri, Jan 26, 2018 at 12:14:40PM +0100, Marek Szyprowski wrote: > >>glibc in calls cacheflush syscall on the whole textrels section of the > >>relocated binaries. However, relocation usually doesn't touch all pages > >>of that section, so not all of them are read to memory when calling this > >>syscall. However flush_cache_user_range() function will unconditionally > >>touch all pages from the provided range, resulting additional overhead > >>related to reading all clean pages. Optimize this by calling > >>flush_cache_user_range() only on the pages that are already in the > >>memory. > >What ensures that another CPU doesn't remove a page while we're > >flushing it? That will trigger a data abort, which will want to > >take the mmap_sem, causing a deadlock. > > I thought that taking mmap_sem will prevent pages from being removed. > mmap_sem has been already taken in the previous implementation of that > syscall, until code simplification done by commit 97c72d89ce0e ("ARM: > cacheflush: don't bother rounding to nearest vma").
No, you're not reading the previous code state correctly. Take a closer look at that commit.
find_vma() requires that mmap_sem is held across the call as the VMA list is not stable without that semaphore held. However, more importantly, notice that it drops the semaphore _before_ calling the cache flushing function (__do_cache_op()).
The point is that if __do_cache_op() faults, it will enter do_page_fault(), which will try to take the mmap_sem again, causing a deadlock.
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