<div dir="ltr">Nod, the volatile just means it can change by itself (which means there might be other hardware setting it) thus the compiler should be read from source each time it is used, verse poked into a register and kept locally, <div><strike><br></strike></div><div><strike>or even more insidious read "early" and precomputed and just used "later"</strike>. </div><div><br></div><div>But now that I type that last bit, this is exactly what the memory barriers are then to force. an instruction to "read memory" can happen anytime prior to "the line of code", and actually "any time prior to when the result is used" (these are allowed by both CPU and compiler) while volatile is just a compiler thing. But also given the CPU has not changed, so we are really looking at the level of aggression of the compiler. Slapping something like full barrier `<span style="color:rgb(0,0,0);font-family:monospace;font-size:medium">__sync_synchronize` in might help </span><a href="https://stackoverflow.com/questions/19965076/gcc-memory-barrier-sync-synchronize-vs-asm-volatile-memory">https://stackoverflow.com/questions/19965076/gcc-memory-barrier-sync-synchronize-vs-asm-volatile-memory</a><br><br><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, 6 Feb 2022 at 15:43, Bevin Brett <<a href="mailto:bevin_brett@hotmail.com">bevin_brett@hotmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div dir="ltr">
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
Assuming you are looking at a memory coherency issue, think of it like this...<br>
</div>
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<br>
CPU0<br>
cache0</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
sharedMemory<br>
cache1<br>
CPU1<br>
<br>
</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
Your old code brought the volatile struct in from disk into cache0 a long time before it was used by CPU1.</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
<br>
</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
<span style="color:rgb(0,0,0);font-family:Calibri,Helvetica,sans-serif;font-size:12pt"> struct inited into cache0 by disk read</span><br>
</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
... long time elapses</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
cache0 version of struct gets flushed to sharedMemory</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
... long time elapses</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
CPU0 tells CPU1 to use struct</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
CPU1 reads sharedMemory to get struct into cache1</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
CPU1 uses the values from struct<br>
<br>
Your new code does it MUCH later</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
<br>
<div style="margin:0px;font-size:12pt"><span style="margin:0px;font-size:12pt"><span style="margin:0px"> struct inited into cache0 by instructions</span></span></div>
<div style="margin:0px;font-size:12pt"> <b>NOTHING FLUSHES</b> cache0 to sharedMemory</div>
<div style="margin:0px;font-size:12pt"> <b>ALMOST NO TIME ELAPSES</b></div>
<div style="margin:0px;font-size:12pt"> CPU0 tells CPU1 to use struct</div>
<div style="margin:0px;font-size:12pt"> CPU1 reads sharedMemory to get struct into cache1</div>
<span style="margin:0px;font-size:12pt"><span style="margin:0px;font-size:12pt;background-color:rgb(255,255,255)"> <span> </span><b>PERHAPS A LONG TIME AFTER THAT READ</b><span> </span>cache0 is flushed sharedMemory</span></span></div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
<span style="margin:0px;font-size:12pt"> CPU1 uses the values from the uninitialized values of struct<br>
</span><br>
</div>
<div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">
or worse<br>
<div style="margin:0px;font-size:12pt"><span style="margin:0px"><span style="margin:0px"><span style="margin:0px"><br>
</span></span></span></div>
<div style="margin:0px;font-size:12pt"><span style="margin:0px"><span style="margin:0px"><span style="margin:0px"> struct inited into cache0 by instructions</span></span></span>
<div style="margin:0px"> <span> </span><b>NOTHING FLUSHES</b><span> </span>cache0 to sharedMemory</div>
<div style="margin:0px"> <span> </span><b>ALMOST NO TIME ELAPSES</b></div>
<div style="margin:0px"><span style="margin:0px;background-color:rgb(255,255,255)"> CPU1 reads something near struct from sharedMemory getting the uninited struct into cache1</span></div>
<div style="margin:0px"> CPU0 tells CPU1 to use struct</div>
<div style="margin:0px"> CPU1 reads sharedMemory from cache1 getting an uninitialized struct</div>
<span style="margin:0px"><span style="margin:0px;background-color:rgb(255,255,255)"> <span style="margin:0px"> </span><b>PERHAPS A LONG TIME AFTER THAT READ</b><span style="margin:0px"> </span>cache0 is flushed sharedMemory</span></span></div>
<span style="margin:0px;font-size:12pt"><span style="margin:0px"> CPU1 uses the values from the uninitialized values of struct<br>
</span></span><br>
Depending on the h/w, memory flushes may be needed to force the CPU0 to write the data to the shared memory, and memory barriers may be needed to stop CPU1 from reading the earlier versions of the data.<br>
<br>
As has been mentioned previously, volatile by itself does not necessarily cause these flushes and barriers to be needed<br>
<br>
</div>
</div>
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