On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote: > On Wed, 19 Dec 2012 06:40:15 -0800 > Guenter Roeck <li...@roeck-us.net> wrote: > > > Commit 263a523 fixes a warning seen with W=1 due to change in > > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations > > with unsigned divisors to unsigned, even if the dividend is signed and > > negative (for example, -10 / 5U = 858993457). The C standard says "If one > > operand has unsigned int type, the other operand is converted to unsigned > > int", so the compiler is not to blame. > > As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return > > bad values, since the automatic conversion of expressions such as "0 - 2U/2" > > to unsigned was not taken into account. > > > > Fix by checking for the divisor variable type when deciding which operation > > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad > > values > > for negative dividends divided by unsigned divisors. Mark the latter case as > > unsupported. > > The changelog didn't describe the end-user visible effects of the bug. > Please always include this information. Because... > One observed effect is that the s2c_hwmon driver reports a value of 4198403 instead of 0 if the ADC reads 0.
Other impact is unpredictable. Problem is seen if the divisor is an unsigned variable or constant and the dividend is less than (divisor/2). > The patch is applicable to 3.7.x. Should we backport it? > Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is unpredictable. 3.6 needs it as well. -- 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/
