Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 07:15:57PM +, Pranith Kumar wrote: > On Fri, Feb 27, 2015 at 2:08 PM, Mark Rutland wrote: > > On Fri, Feb 27, 2015 at 06:44:19PM +, Pranith Kumar wrote: > >> On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas > >> wrote: > >> > It's either badly formatted or I don't get it. Are the "stxr x1" and > >> > "stxr x7" happening on the same CPU (P0)? If yes, that's badly written > >> > code, not even architecturally compliant (you are not allowed other > >> > memory accesses between ldxr and stxr). > >> > >> OK. Is that the same case with ldaxr (acquire) and stlxr (release)? > >> AFAIK, memory accesses between acquire and release exclusive > >> operations are allowed. > > > > The restriction on memory accesses in the middle of a load-exclusive > > store-exclusive sequence applies to all the load/store-exclusive > > variants, including ldaxr and stlxr. > > > > Thanks Mark. I am trying to see where this restriction is documented. > Looking at: > http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/a64_data_transfer_alpha.html > I do not see that mentioned. The only restriction relevant is that a > stxr should use the same address as the most recent ldxr. > > Could you please point me to the relevant documentation? You will need to look at the latest ARMv8-A Architecture Reference Manual [1]. Table B2-2 defines the set of Load-Exclusive and Store-Exclusive variants, including LDAXR and STLXR. Taking this into account, take a look at the restrictions in section B2.10.5 "Load-Exclusive and Store-Exclusive instruction usage restrictions". One of the bullet points nodes that software must avoid explicit memory accesses between a Load-Exclusive instruction and the associated Store-Exclusive. Thanks, Mark. [1] http://infocenter.arm.com/help/topic/com.arm.doc.ddi0487a.e/index.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 2:08 PM, Mark Rutland wrote: > On Fri, Feb 27, 2015 at 06:44:19PM +, Pranith Kumar wrote: >> On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas >> wrote: >> > It's either badly formatted or I don't get it. Are the "stxr x1" and >> > "stxr x7" happening on the same CPU (P0)? If yes, that's badly written >> > code, not even architecturally compliant (you are not allowed other >> > memory accesses between ldxr and stxr). >> >> OK. Is that the same case with ldaxr (acquire) and stlxr (release)? >> AFAIK, memory accesses between acquire and release exclusive >> operations are allowed. > > The restriction on memory accesses in the middle of a load-exclusive > store-exclusive sequence applies to all the load/store-exclusive > variants, including ldaxr and stlxr. > Thanks Mark. I am trying to see where this restriction is documented. Looking at: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/a64_data_transfer_alpha.html I do not see that mentioned. The only restriction relevant is that a stxr should use the same address as the most recent ldxr. Could you please point me to the relevant documentation? Thanks! -- Pranith -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 06:44:19PM +, Pranith Kumar wrote: > On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas > wrote: > > It's either badly formatted or I don't get it. Are the "stxr x1" and > > "stxr x7" happening on the same CPU (P0)? If yes, that's badly written > > code, not even architecturally compliant (you are not allowed other > > memory accesses between ldxr and stxr). > > OK. Is that the same case with ldaxr (acquire) and stlxr (release)? > AFAIK, memory accesses between acquire and release exclusive > operations are allowed. The restriction on memory accesses in the middle of a load-exclusive store-exclusive sequence applies to all the load/store-exclusive variants, including ldaxr and stlxr. Thanks, Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas wrote: > It's either badly formatted or I don't get it. Are the "stxr x1" and > "stxr x7" happening on the same CPU (P0)? If yes, that's badly written > code, not even architecturally compliant (you are not allowed other > memory accesses between ldxr and stxr). OK. Is that the same case with ldaxr (acquire) and stlxr (release)? AFAIK, memory accesses between acquire and release exclusive operations are allowed. > >> The last store exclusive succeeds since the exclusive bit is set which >> should not happen. Clearing the exclusive bit before returning from cmpxchg >> prevents this happening. >> >> Now I am not sure how likely this will happen. One can argue that a cmpxchg() >> will not happen between an external ldxr/stxr. But isn't clearing the >> exclusive >> bit better? > > The only way cmpxchg() could happen between a different ldxr/stxr is > during an interrupt. But ERET automatically clears the exclusive > monitor, so the "stxr x7" would not succeed. That makes sense. But please consider the ldaxr/stlxr case and let me know. Thanks! -- Pranith -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 06:25:25PM +, Pranith Kumar wrote: > On Fri, Feb 27, 2015 at 5:06 AM, Will Deacon wrote: > > On Fri, Feb 27, 2015 at 05:46:55AM +, Pranith Kumar wrote: > >> In cmpxchg(), we do a load exclusive on an address and upon a comparison > >> fail, > >> we skip the store exclusive instruction. This can result in the exclusive > >> bit > >> still set. If there was a store exclusive after this to the same address, > >> that > >> will see the exclusive bit set. This should not happen. > > > > ... and the problem with that is? > > Consider the following scenario: > > P0P1 > - > ldxr x7, [B] // exclusive bit set > add x7, x7, #1 > str ..., [B] // exclusive bit cleared > cmpxchg: > ldxr x0, [B] // exclusive bit set > cmp x0, #0// cmp fails > b.ne 1f // branch taken > stxr x1, [B] // end of cmpxchg > 1: > stxr x7, [B]// succeeds? It's either badly formatted or I don't get it. Are the "stxr x1" and "stxr x7" happening on the same CPU (P0)? If yes, that's badly written code, not even architecturally compliant (you are not allowed other memory accesses between ldxr and stxr). > The last store exclusive succeeds since the exclusive bit is set which > should not happen. Clearing the exclusive bit before returning from cmpxchg > prevents this happening. > > Now I am not sure how likely this will happen. One can argue that a cmpxchg() > will not happen between an external ldxr/stxr. But isn't clearing the > exclusive > bit better? The only way cmpxchg() could happen between a different ldxr/stxr is during an interrupt. But ERET automatically clears the exclusive monitor, so the "stxr x7" would not succeed. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 5:06 AM, Will Deacon wrote: > On Fri, Feb 27, 2015 at 05:46:55AM +, Pranith Kumar wrote: >> In cmpxchg(), we do a load exclusive on an address and upon a comparison >> fail, >> we skip the store exclusive instruction. This can result in the exclusive bit >> still set. If there was a store exclusive after this to the same address, >> that >> will see the exclusive bit set. This should not happen. > > ... and the problem with that is? Consider the following scenario: P0P1 - ldxr x7, [B] // exclusive bit set add x7, x7, #1 str ..., [B] // exclusive bit cleared cmpxchg: ldxr x0, [B] // exclusive bit set cmp x0, #0// cmp fails b.ne 1f // branch taken stxr x1, [B] // end of cmpxchg 1: stxr x7, [B]// succeeds? The last store exclusive succeeds since the exclusive bit is set which should not happen. Clearing the exclusive bit before returning from cmpxchg prevents this happening. Now I am not sure how likely this will happen. One can argue that a cmpxchg() will not happen between an external ldxr/stxr. But isn't clearing the exclusive bit better? -- Pranith -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail
On Fri, Feb 27, 2015 at 05:46:55AM +, Pranith Kumar wrote: > In cmpxchg(), we do a load exclusive on an address and upon a comparison fail, > we skip the store exclusive instruction. This can result in the exclusive bit > still set. If there was a store exclusive after this to the same address, that > will see the exclusive bit set. This should not happen. ... and the problem with that is? > We need to clear the exclusive bit in these cases. Why? Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
