Re: [patch] Re: using long instead of atomic_t when only set/read is required

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From: Linus Torvalds
Date: Monday, March 3, 2008 - 10:26 am

On Mon, 3 Mar 2008, Alan Stern wrote:

Yes, we do depend on this.  All the RCU stuff (and in general *anything* 
that depends on memory ordering as opposed to full locking, and we have 
quite a lot of it) is very fundamentally dependent on the fact that things 
like pointers get read and written atomically.

HOWEVER, it is worth pointing out that it's generally true in a 
"different" sense than the actual atomic accesses. For example, if you 
test a single bit of a word, it's still quite possible that gcc will have 
turned that "atomic" read into a single byte read, so it's not necessarily 
the case that we'll actually even read the whole word. 

(Writes are different: if you do things like bitwise updates they simply 
*will*not* be atomic, but that's simply not what we depend on anyway).

So in that sense, the atomicity guarantees are a lot weaker than the ones 
we do for IO accesses, but that's all fine. Memory isn't IO, and doesn't 
have side effects.

			Linus
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Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Wed Feb 20, 1:58 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Wed Feb 20, 3:15 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Wed Feb 20, 3:41 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Wed Feb 20, 5:02 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Thu Feb 21, 9:38 am)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Thu Feb 21, 3:47 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Fri Feb 22, 6:30 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sat Feb 23, 1:16 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sat Feb 23, 5:19 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 6:33 am)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 6:51 am)
Re: [linux-pm] [Bug 10030] Suspend doesn't work when SD ca ..., Rafael J. Wysocki, (Sun Feb 24, 7:00 am)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 1:09 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 1:25 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 1:56 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 3:18 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Sun Feb 24, 3:21 pm)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Mon Feb 25, 4:40 am)
Re: [Bug 10030] Suspend doesn't work when SD card is inserted, Rafael J. Wysocki, (Mon Feb 25, 4:41 am)
Re: [patch] Re: using long instead of atomic_t when only s ..., Linus Torvalds, (Mon Mar 3, 10:26 am)